1) Field of the Invention
The subject invention relates to a method of purifying polyether polyols of non-volatile impurities.
2) Description of Related Art
Propylene oxide is a valuable precursor for the preparation of polyether polyols, which are employable in the production of polyurethane foams by reaction with an isocyanate. In general, commercially available propylene oxide for this purpose is of high purity, substantially all impurities having been removed prior to commercial availability by subjecting the propylene oxide product to various purification techniques involving distillation and/or separation from other components produced in its preparatory reaction or introduced in the purification of the propylene oxide product itself.
Techniques disclosed in the related art are extensive to put the propylene oxide in this commercially acceptable form. Most of these processes include distillation, fractionation, or separation, of propylene oxide in the presence of a hydrocarbon. Although a number of techniques, employed alone or in combination, have been successful in the production of commercially acceptable propylene oxide products, it was noted that certain polyurethane foams based on polyether polyols derived from such commercially available propylene oxide compositions would sometimes collapse even though the raw materials employed in the production of such polyurethane foam product, including intermediates therefore, would meet acceptable specifications.
The foam collapse was attributable to the presence of certain nonvolatile impurities present in the propylene oxide starting material employed in the preparation of the polyether polyol. Hence, even after the obtaining propylene oxide which meets commercially accepted specifications, the ultimate commercial acceptability of the propylene oxide was unpredictable due to the presence of these nonvolatile impurities which led to unacceptable polyurethane foam products exhibiting unstable foam rise, foam collapse, and/or substantial internal blow holes. This is attributed to the nonvolatile impurity, namely, poly(propylene oxide) (PPO), which has a molecular weight of at least 50,000. This impurity can make the propylene oxide unsuitable for making polyether polyols to be used with an isocyanate in the manufacture of acceptable polyurethane foams in certain formulations.
The purification process is typically followed by prompt conversion of the propylene oxide (PO) into the desired polyether polyols. However, prior to conversion, the PO can be shipped or stored in a vessel, which is made of conventional carbon steel. Conventional carbon steels can catalyze, over a period of time and at a given temperature, the formation of the undesired high molecular weight PPO impurity.
These commercially available propylene oxide products produce polyether polyols that yield unstable polyurethane foams having blow holes and/or low foam rise, even though conventional analysis of such polyether polyol products has failed to reveal the presence of contaminants that would be indicative of poor quality foam products. It has been determined that polyether polyols made from propylene oxide having reduced levels of the high molecular weight PPO impurity produced polyurethane foams with improved foam rise and with fewer blow hole formation.
Therefore, processes were developed for the purification and/or separation of propylene oxide even after being in an acceptable commercial state. U.S. Pat. No. 4,692,535 discloses a processes starting with the propylene oxide product suitable as an intermediate in the production of polyether polyols for forming polyurethane foam. The PPO was substantially removed from an otherwise commercially acceptable propylene oxide, prior to forming the polyether polyol for use in forming the polyurethane foam.
The '535 patent discloses filtering or percolating the PO through a fixed bed of an adsorbent material. The adsorbent materials included activated carbon, charcoal and attapulgite, either in granular or powdered forms. The quantities of adsorbent to be used per unit volume of PO to be treated are estimated at from about 0.001 to about 0.01 grams or more of solid adsorbent per gram of propylene oxide, with contact times ranging from about 1 to about 15 minutes, temperatures of from about 10 to about 100° C. and pressures ranging from atmospheric to super-atmospheric.
Another process was described in U.S. Pat. No. 5,235,075, to purify propylene oxide containing an unacceptable quantity of PPO. The process comprises contacting the propylene oxide with a diatomaceous earth for a time and under conditions sufficient to reduce the amount of contaminant to acceptable levels. Again, the purification was of the propylene oxide itself and not of a formed polyether polyol.
Accordingly, it would be advantageous to provide a method for purifying a polyether polyol of non-volatile impurities after the polyether polyol has been formed. While the prior art purifies the propylene oxide prior to forming the polyether polyol, the propylene oxide may still be contaminated before incorporation into the polyether polyol. Therefore, if the polyether polyol was purified, instead of the starting components, there would be a less likelihood of contamination leading to unstable foam products. Further, it would be advantageous to provide a method of purifying the polyether polyol without suffering large material losses during the purification and filtering thereof. And given that only certain foam formulations of given polyether polyol products are substantially influenced by the presence of PPO, a system that could be utilized on an as needed basis would also be advantageous.